What is the threshold where you would have to check baggage instead of going carryon only? Excluding special trip specific items that are too large or not allowed to be carried on.

By "threshold", consider length of trip, weather at destination, etc..

I have done a two week warm weather trip with just a carryon, but added a (small) checked bag for a two week cooler weather trip. In both cases, I did wash some clothes in the sink / bathtub.

For me its got more to do with the journey on the ground. If I'm being picked up by car and being driven to a hotel etc., then I'll happily check-in a bigger suitcase. If I know I'm going to be battling public transport, then I'll travel lighter.

Usually I will check luggage because I like to take a pen knife, "some liquids" etc., and even if not, I might want to bring something back of that ilk.

I've never lost a checked bag, only once had one delayed, and twice had suitcases damaged (though not seriously). So I never fear checking baggage. Frankly it annoys me when I have to wait so long at a gate x-ray because people ahead of me are pushing the limits so much - roll-on, separate laptop bag, plastic bags with duty free, handbags, etc. It also increases boarding time as pax block the aisles whilst trying to find somewhere to stow all this stuff.

To answer your specific point - I suppose it has little to do with length of stay, weather etc. Except that if its a very short trip (ie day trip, or one night away) then its unlikely I would check luggage. Mind you, I did a day trip to London from Saigon and managed to check 42Kg (about 92lbs)

The only times I've checked baggage in the past twenty years have been when I was traveling with either (a) an item, not luggage, that wouldn't fit in the overhead rack and had to be checked; or (b) a travel companion who couldn't fit her stuff into something she could carry on, so as long as we'd have to wait for her bag, I figured we might as well wait for mine too. (Sometimes a relationship is worth that sacrifice.) That includes trips of up to a month - and on that one, my carry-on also had to hold a bicycle helmet.